In general, in as electric-power conversion apparatus including a three-phase voltage-type inverter to be PWM (Pulse Width Modulation)-controlled, turning-on/off of switching devices provided in the upper arm and the lower arm of each phase is PWM-controlled, based on the output current value (referred to as a current detection value, hereinafter) of each phase, of the electric-power conversion apparatus, that is detected by a current detector; thus, the voltage is controlled in such a way as to keep track of a voltage command value.
In the foregoing current detector, the reference electric potential that is common in the respective phases out of the three phases is set to the ground potential, in general. Accordingly, noise that intrudes in the current detection value of each phase includes much components that are in-phase among the respective phased out of the three phase, i.e., zero-phase components.
In this situation, in the region where the amplitude of the foregoing voltage command value is small, the foregoing current detector can obtain the respective current detection values of all the phases out of the three phases. Thus, when the respective current detection values of all the phases out of three phases are converted into, for example, two axes in a stationary biaxial coordinate system or a rotating biaxial coordinate system, no zero-phase component included in the current detection value appears; therefore, it is made possible that without undergoing the effect of the zero-phase components included in noise, high-accuracy voltage control can be performed based on the current detection value corresponding to an output current flowing in the three-phase voltage-type inverter.
However, in the case where in the region where the amplitude of the forgoing voltage command value is large, the respective phases out of the three phases are tentatively referred to as “a maximum phase”, “a middle phase”, and “a minimum phase” in decreasing order in size of the voltage command values for the respective phases out of the three phases, the time in which the switching device in the lower arm of the maximum phase is turned on is shorter than a time necessary for the current detector to detect a correct current detection value; thus, because the current detector cannot detect the correct, current detection value of the maximum phase, the foregoing high-accuracy voltage control based on the current detection value may not be performed.
To date, there has been disclosed a technology in which in a three-phase voltage-type inverter to be PWM-controlled, there are selected, every one period of a carrier wave in the PWM control, any two phases, in each of which the off time, based on the PWM control, of the lower arm is shorter than the off times of the other phases, in which the current detector concurrently detects energization currents to the respective lower arms of the selected two phases, and in which the current detection values, as the output current values of the inverter, are utilized in the PWM control (for example, refer to Patent Document 1).
It is allegedly claimed that the conventional three-phase voltage-type inverter disclosed in Patent Document 1 makes it possible that the energization currents of two suitable phases are concurrently detected every predetermined period and that the values based on the current detection values are dealt with, for example, as the instantaneous current vectors in vector control of an AC motor.